1. Field of the Invention
The present invention relates to a process for producing highly pure terephthalic acid, and more in detail, the present invention relates to a process for producing highly pure terephthalic acid which can be directly reacted with a glycol component to produce polyesters.
2. Description of the Prior Art
Terephthalic acid is useful as a starting material for producing polyesters, and is ordinarily produced by the so-called SD process wherein p-xylene is brought into reaction with molecular oxygen in the presence of a catalyst containing heavy metal(s) and bromine. However, since 1,000 to 3,000 ppm of 4-carboxybenzaldehyde (hereinafter referred to as 4CBA) is contained as an impurity in the terephthalic acid produced by SD process, it cannot be used as a staring material for producing polyesters used for preparing filaments, films and the like, as it is.
Accordingly, a method has hitherto been adopted, wherein crude terephthalic acid is brought into reaction with methanol to form dimethyl terephthalate, and after purifying thereof, the terephthalate is brought into reaction with a glycol component, or crude terephthalic acid is dissolved in an aqueous solvent at a high temperature and under a high pressure and then the solution is brought into contact with a noble metal catalyst for purification, thereby obtaining the purified terephthalic acid as the starting material for producing polyesters.
Namely, there has been a problem in the above-mentioned processes that in addition to the plant for producing crude terephthalic acid by SD process, it is necessary to provide another plant for purifying the crude terephthalic acid separately.
Accordingly, in recent years, it has become possible to directly produce highly pure terephthalic acid in a single plant by carrying out the oxidation of p-xylene in the presence of a specified catalyst under specified conditions of oxidation, or in a specified oxidation process.
Formerly, the present applicant has proposed an advantageous process for directly producing highly pure terephthalic acid with a content of 4CBA of not more than 500 ppm in a single plant, wherein a reaction mixture which has been obtained by oxidizing p-xylene and contains terephthalic acid is further oxidized subsequently at a temperature lower than that of the first oxidation step and the thus obtained reaction product is further oxidized at a temperature higher than 235.degree. C. (refer to U.S. Pat. No. 4,286,101). The process disclosed in U.S. Pat. No. 4,286,101 is industrially advantageous because of the preparation of highly pure terephthalic acid in a single plant and of the reduced loss of acetic acid as a solvent during the production of terephthalic acid.
Although terephthalic acid obtained by the above-mentioned process is satisfactorily usable as a commercial starting material for producing polyesters for use in preparing, for instance, fibers, films and the like because of its low content of 4CBA and its high transmissivity, as a result of the present inventors' studies for processes of obtaining terephthalic acid of a higher quality, they have found that terephthalic acid obtained from a specified treatment of the reaction mixture obtained by the third step of oxidation forms the polyesters particularly excellent in its colour tone in spite of the fact that the content of 4CBA and transmissivity are nearly the same with the above-mentioned process, and have been attained to the present invention.
Namely, the object of the present invention is to provide a process for continuously producing highly pure terephthalic acid which gives polyesters excellent in its colour tone.